Optical transceiver for controlling steering angle between receive light beam and transmit light beam

ABSTRACT

An optical transceiver for controlling a steering angle between a receive light beam and a transmit light beam includes an optical beam coupling device. The optical beam coupling device comprises a plurality of optical elements configured to control a steering angle between the receive light beam received by the optical beam coupling device along a first line of sight (LOS) and the transmit light beam that is output from the optical beam coupling device along a second LOS different from the first LOS, wherein both the receive light beam and the transmit light beam pass through the plurality of optical elements. The plurality of optical elements have a set of combinations for different positions of each of the optical elements, wherein each position in the set of combinations induces a different steering angle between the transmit light beam and the receive light beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/995,457, filed Jun. 1, 2018, which claims benefit of U.S. ProvisionalPatent Application Ser. No. 62/649,496, filed Mar. 28, 2018, which arehereby incorporated by reference in their entirety.

BACKGROUND

The present disclosure generally relates to a laser communicationsystem, and specifically relates to an optical transceiver forcontrolling a steering angle between a receive light beam and a transmitlight beam.

Laser-based systems, such as laser communication systems, commonlyemploy multiple laser beams. A bidirectional laser communication system(e.g., in space settings) can include two or more laser terminals (e.g.,either stationary or non-stationary terminals) that communicate betweeneach other by encoding information into light beams. To exchangeinformation data between two laser terminals of the laser communicationsystem, each laser terminal transmits a light beam with encoded data toanother laser terminal and receives another light beam with encoded datatransmitted from the other laser terminal.

If a relative velocity between the laser terminals is low (e.g., below athreshold velocity), the light beam transmitted from the laser terminaland other light beam received from the other light terminal aresubstantially aligned along the same line of sight (LOS). On the otherhand, if the relative velocity between the laser terminals is above thethreshold velocity (which often happens in ground-space andspace-to-space terminal communications), the relativistic effect needsto be accounted for when the laser terminals communicate between eachother. Therefore, for accurate communication between the laserterminals, it is required to differentiate between a position of theother laser terminal relative to the laser terminal when a light beam isreceived from the other light terminal and another future position ofthe other laser terminal relative to the laser terminal for directinganother light beam toward the other light terminal. Typically, the lightbeam received from the other laser terminal and the other light beamtransmitted to the other light terminal are not parallel to each other.Instead, from the perspective of the laser terminal, there is a certaindifferential (point-ahead) angle between transmit and receive lightbeams.

The conventional approach is to implement two very precise separatepointing systems at the laser terminal. A first pointing system (e.g.,receiving pointing system) is required to direct the receive light beamtoward a detector of the laser terminal. A second pointing system (e.g.,transmitting pointing system) implemented separately from the firstpointing system needs to be configured to provide a differential(point-ahead) angle to send the transmit light beam to the other laserterminal accounting for a relative velocity and separation between thelaser terminals. Accordingly, the conventional approach is relativelycomplex, as it uses two separate pointing systems to accurately aligntransmit and receive light beams that must be precisely aligned to eachother.

SUMMARY

An optical transceiver for controlling a steering angle between areceive light beam and a transmit light beam is presented herein. Theoptical transceiver may be implemented as part of a laser terminal of alaser communication system that communicates with another (remote) laserterminal. The optical transceiver comprises an optical beam couplingdevice that includes a plurality of optical elements. The opticalelements are configured to control a steering angle between the receivelight beam received by the optical beam coupling device along a firstline of sight (LOS) and the transmit light beam that is output from theoptical beam coupling device along a second LOS different from the firstLOS. Both the receive light beam and the transmit light beam passthrough the plurality of optical elements. The plurality of opticalelements have a set of combinations for different positions of each ofthe optical elements, wherein each position in the set of combinationsinduces a different steering angle between the transmit light beam andthe receive light beam. In some embodiments, the optical transceiverfurther comprises a controller coupled to the optical beam couplingdevice. The controller estimates a point-ahead angle between the firstLOS and the second LOS, and controls a position of each of the pluralityof optical elements to steer at least one of the transmit light beam andthe receive light beam relative to each other, based on the estimatedpoint-ahead angle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example laser communication system comprising twocommunicating laser terminals, in accordance with one or moreembodiments.

FIG. 2A is a laser terminal including an optical transceiver forcontrolling a steering angle between a receive light beam and a transmitlight beam, in accordance with one or more embodiments.

FIG. 2B is an example of an optical beam coupling device of the opticaltransceiver of FIG. 2A having a plurality of optical elements, inaccordance with one or more embodiments.

FIG. 3A is an example of an optical beam coupling device of the opticaltransceiver of FIG. 2A implemented using a pair of compound prisms, inaccordance with one or more embodiments.

FIG. 3B is another example of an optical beam coupling device of theoptical transceiver of FIG. 2A implemented using a pair of compoundprisms, in accordance with one or more embodiments.

FIG. 4 is an example of an optical beam coupling device of the opticaltransceiver of FIG. 2A implemented using a pair of quarter-waveplatescoupled to a pair of compound prisms, in accordance with one or moreembodiments.

FIG. 5 is a flow chart illustrating a process of controlling a steeringangle between a receive light beam and a transmit light beam, which maybe implemented at the laser terminal shown in FIG. 2A, in accordancewith one or more embodiments.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles, or benefits touted, of the disclosure described herein.

DETAILED DESCRIPTION

The teachings described herein may be used in laser communicationsystems. The teachings herein may be incorporated into (e.g.,implemented within or performed by) laser terminals of a lasercommunication system. The laser terminals of the laser communicationsystem communicate between each other using laser beams, i.e., lightbeams.

FIG. 1 is an example laser communication system 100 comprising two laserterminals 105, 110 that communicate with each other using light beams,in accordance with one or more embodiments. In some embodiments, thelaser communication system 100 is a ground-space communication system,e.g., the laser terminal 105 may be a stationary laser terminal locatedon the Earth and the laser terminal 110 may be a non-stationary laserterminal placed on a satellite orbiting the Earth. In some otherembodiments, the laser communication system 100 is a space-to-spacecommunication system, i.e., both laser terminals 105, 110 are located inspace, e.g., located on different satellites orbiting the Earth. Ingeneral, at least one of the laser terminals 105, 110 is anon-stationary laser terminal, and there is a certain relative velocitybetween the laser terminal 105 and the laser terminal 110. In someembodiments, the laser terminals 105, 110 are far apart from each other,and the laser terminals 105, 110 have high velocities relative to eachother, e.g., above a threshold velocity. Because of that, there is acertain differential (point-ahead) angle between a light beamtransmitted from each laser terminal 105, 110 and another light beamreceived by that laser terminal 105, 110.

In the illustrative embodiment of FIG. 1, the laser terminal 105 isshown as a stationary laser terminal, whereas the laser terminal 110 isshown as a non-stationary laser terminal. However, both laser terminals105, 110 can be non-stationary laser terminals. As shown in FIG. 1,there is a relative velocity 113 between the laser terminal 105 and thelaser terminal 110. At a time instant t₁, the laser terminal 110 may beat a first position relative to the laser terminal 105. The laserterminal 105 receives a receive light beam 115 transmitted from thelaser terminal 110 at the time instant t₁. At some other time instant t₂in the future, the laser terminal 110 will be at a new second positionrelative to the laser terminal 105, wherein the second relative positionis based on the relative velocity 113. The laser terminal 105 needs toaccount for changing of relative positions between the laser terminal105 and the laser terminal 110 when sending a light beam to the laserterminal 110. Thus, the laser terminal 110 outputs a transmit light beam120 toward a future relative position of the laser terminal 110, i.e.,the laser terminal 105 steers the transmit light beam 120 from thereceive light beam 115 by a certain differential (point-ahead) angle125. In other words, the receive light beam 115 is received by the laserterminal along a first line of sight (LOS) and the transmit light beam120 is output from the laser terminal 105 along a second LOS differentfrom the first LOS. The difference between the first LOS and the secondLOS is referred to as the point-ahead angle 125. Note that thepoint-ahead angle 125 may be a function of the relative velocity 113 anda distance between the laser terminals 105, 110. The point-ahead angle125 can be referred to herein as a steering angle.

An optical transceiver presented in this disclosure for implementationin a laser terminal (e.g., in any of the laser terminals 105, 110)allows for precise control of a steering angle between a transmit lightbeam and a receive light beam. An optical beam coupling device for theoptical transceiver represents a single mode fiber device that induces aspecific steering angle between the transmit light beam and the receivelight beam. Unlike two separate optical assemblies in conventional laserterminals, the optical beam coupling device presented herein provides asingle optical assembly that is used by both the transmit light beam andthe receive light beam. The optical beam coupling device includes aplurality of optical elements to induce differential steering betweentransmit and receive beams, thereby allowing dynamic control of apoint-ahead angle using the single optical assembly. In someembodiments, the optical elements of the optical beam coupling devicemay be, e.g., compound prisms, wedges, diffraction gratings, or somecombination thereof. Wavelengths and/or polarization of light may beused to differentiate between transmit and receive light beams, asdisclosed in more detail in conjunction with FIGS. 3A-3B, FIG. 4.

FIG. 2A is an example laser terminal 200 including an opticaltransceiver 205 for controlling a steering angle between a receive lightbeam 210 received by the optical transceiver 205 and a transmit lightbeam 215 output by the optical transceiver 205, in accordance with oneor more embodiments. The laser terminal 200 may be an embodiment of thelaser terminal 105 of FIG. 1. The optical transceiver 205 is configuredto in-couple the receive light beam 210 transmitted from another laserterminal (not shown in FIG. 2A) and to out-couple the transmit lightbeam 215 to the other laser terminal, by inducing a steering angle 220between the receive light beam 210 and the transmit light beam 215 toaccount for a relative velocity between the laser terminal 200 and theother laser terminal. The optical transceiver 205 includes an opticalbeam coupling device 225 coupled to a transceiver fiber 227, and acontroller 235 coupled to the optical beam coupling device 225.

The optical beam coupling device 225 is a single optical assembly usedby both the receive light beam 210 and the transmit light beam 215 toinduce a specific steering angle 220 (i.e., point-ahead angle) betweenthe receive light beam 210 and the transmit light beam 215. The opticalbeam coupling device 225 includes a plurality of optical elementsconfigured to induce differential steering between the receive andtransmit light beams 210, 215. More details about structure andoperation of the optical beam coupling device 225 are disclosed inconjunction with the FIGS. 2B, 3A-3B, FIG. 4.

The transceiver fiber 227 is coupled to the optical beam coupling device225. The transceiver fiber 227 may be configured to provide the transmitlight beam 215 that is being passed to the optical beam coupling device225 for differential steering. The transceiver fiber 227 may be furtherconfigured to in-couple the receive light beam 210 being differentiallysteered by the optical beam coupling device 225. In some embodiments,the transceiver fiber 227 includes a detector for sensing an intensitysignal associated with the receive light beam 210.

The controller 229 may be coupled to at least one optical element of theplurality of optical elements of the optical beam coupling device 225,e.g., via one or more positioners (not shown in FIG. 2A). The controller229 may control a position of the at least one optical element of theoptical beam coupling device 225 to achieve a specific steering angle220 between the receive light beam 210 and the transmit light beams 215.The controller 229 may estimate a point-ahead angle (i.e., steeringangle 220) between a first LOS along which the receive light beam 210was received by the optical beam coupling device 225 and a second LOSalong which the transmit light beam 215 will be output from the opticalbeam coupling device 225. The controller 229 may estimate thepoint-ahead angle based on, e.g., a relative velocity between the laserterminal 200 and the other laser terminal and a distance between thelaser terminal 200 and the other laser terminal. The controller 229 maycontrol a position of the at least one optical element of the opticalbeam coupling device 225 to steer at least one of the transmit lightbeam 215 and the receive light beam 210 relative to each other, based onthe estimated point-ahead angle.

In some embodiments, the transceiver fiber 227 includes a fiber tap (notshown in FIG. 2A) for monitoring a power associated with the receivelight beam 210. Information about the power of receive light beam 210may be provided to the controller 229 interfaced with the transceiverfiber 227. The controller 229 may adjust the steering angle 220, basedon the information about power. If the power starts to fall off (e.g.,the power is below a threshold level), the controller 229 adjusts thesteering angle 220 to bring the power back up. Alternatively, instead ofthe fiber tap, the optical transceiver includes a beam-splitter (notshown in FIG. 2A) that splits of a defined amount of power from thereceive light beam 210 (e.g., 5%, or some other amount that depends on asignal strength of the receive light beam 210). Then, the controller 229coupled to the beam-splitter may be configured to directly measure thesplit off power before the receive light beam 210 is coupled into thetransceiver fiber 227, and to adjust accordingly the steering angle 220.

FIG. 2B illustrates the optical beam coupling device 225 of FIG. 2Ahaving a plurality of optical elements, in accordance with one or moreembodiments. In the illustrative embodiment shown in FIG. 2B, theoptical beam coupling device 225 includes a first optical element 230coupled to a second optical element 235. Although not shown in FIG. 2B,the optical beam coupling device 225 may include one or more additionaloptical elements coupled to the first and second optical elements 230,235. Each optical element 230, 235 is configured to refract an incidentlight beam by a steering angle of a variable amount relative to anoptical axis 240. A steering angle provided by that optical element 230,235 may depend on a structure of incident light (e.g., wavelength,polarization) and a structure of that optical element 230, 235 (e.g.,refraction index of at least one material used for implementation ofthat optical element 230, 235). Note that refraction of light can alsorefer in this disclosure to deflection of light, steering of light, ordiffraction of light.

As shown in FIG. 2B, the first optical element 230 of the optical beamcoupling device 225 is configured to steer a transmit light beam 245relative to the axis 240 by an angle 253, and the second optical element235 is configured to steer the transmit light beam 245 relative to theaxis 240 by an angle 255. Thus, the transmit light beam 245 is outputfrom the optical beam coupling device 225 as a light beam steered fromthe original direction parallel to the axis 240 by an angle equal to thesum of steering angles provided by the first and second optical elements230, 235, e.g., by the sum of the angles 253 and 255. Similarly, thesecond optical element 235 is configured to steer a receive light beam250 relative to the axis 240 by an angle 257, and the first opticalelement 230 is configured to steer the receive light beam 250 relativeto the axis 240 by the angle 260. Thus, the receive light beam 250 issteered by the optical beam coupling device 225 by an angle equal to thesum of steering angles provided by the first and second optical elements230, 235, e.g., the sum of the angles 257 and 260. Thus, the receivelight beam 250 is received by the optical beam coupling device 225 alonga first LOS, and the transmit light beam 245 is output from the opticalbeam coupling device 255 along a second LOS different from the firstLOS. Both the receive light beam 250 and the transmit light beam 245pass through the first and second optical elements 230, 235.

At least one of the first and second optical elements 230, 235 canchange position (e.g., nominally rotate) relative to the axis 240 todynamically adjust a steering angle provided by the at least one of thefirst and second optical elements 230, 235. In this manner, differentsteering angles between transmit and receive light beams can beachieved. In some embodiments, as discussed, the controller 235 isconfigured to control a position of the at least one of the opticalelements 230, 235 to steer at least one of the transmit light beam andthe receive light beam relative to each other, based on a knownpoint-ahead angle. The first optical element 230 and the second opticalelement 235 have a set of combinations for different positions of eachof the first and second optical elements 230, 235. Each position in theset of combinations may induce a different steering angle between thetransmit light beam 245 and the receive light beam 250.

In some embodiments, each of the first and second optical elements 230,235 of the optical beam coupling device 225 are implemented as acompound prism. FIG. 3A is an example of an optical beam coupling device300 implemented using a pair of compound prisms 305, 310, in accordancewith one or more embodiments. The optical beam coupling device 300 maybe an embodiment of the optical beam coupling device 225 of the opticaltransceiver 205 from FIG. 2A; and the compound prisms 305, 310 may beembodiments of the first and second optical elements 230, 235. Althoughnot shown in FIG. 3A, the optical beam coupling device 300 may includeone or more additional compound prisms coupled to the compound prisms305, 310. A compound prism is an optical assembly comprising at least apair of optical elements attached to each other (e.g., prisms or wedges)configured to refract an incident light beam by an angle that depends ona structure of light (e.g., wavelength, polarization) and a structure ofeach optical element in the compound prism (e.g., refraction index of amaterial of each optical element in the compound prism).

As shown in FIG. 3A, each compound prism 305, 310 is composed of a pairof wedges attached to each other, i.e., the compound prism 305 includesa wedge 307 attached to a wedge 309 and the compound prism 310 includesa wedge 311 attached to a wedge 313. The wedges 307, 309 of the compoundprism 305 are attached to each other such that the wedges 307, 309cannot move relative to each other. Similarly, the wedges 311, 313 ofthe compound prism 310 cannot move relative to each other. In theillustrative embodiment of FIG. 3A, each compound prism 305, 310 steersa transmit light beam 315 of a first wavelength without affectingpropagation of a receive light beam 320 of a second wavelength differentfrom the first wavelength. In general, each compound prism 305, 310refracts (steers) the transmit light beam 315 by a first angle andrefracts (steers) the receive light beam 320 by a second angle relativeto an axis 325, wherein wavelengths of the transmit light beam 315 andthe receive light beam 320 are different. Thus, as result, the receivelight beam 320 is received by the optical beam coupling device 300 alonga first LOS, and the transmit light beam 315 is output from the opticalbeam coupling device 300 along a second LOS different from the firstLOS. Both the receive light beam 320 and the transmit light beam 315pass through the compound prisms 305, 310.

Note that, in conventional systems, Risley prisms can be used forsteering of monochromatic beams, wherein two or more prisms are rotatedaround a direction of light propagation. The achromatized Risley prismscan be used for steering wideband light beams incident to the Risleyprisms, i.e., for steering of light beams having different waveleghts. Acompound prism composed of, e.g., a pair of Risley prisms is designed tominimize differential steering between the incident light beams ofdifferent wavelengths. In general, the compound prism made of Risleyprisms provides a fixed angle between two incident light beams ofdifferent wavelengths independent of position of the compound prismrelative to a direction of light propagation. In contrast, compoundprisms presented in this disclosure (e.g., the compound prisms 305, 310)dynamically change a steering angle between a pair of light beams ofdifferent wavelengths, based on different positions of the compoundprisms relative to a direction of light propagation (e.g., the axis325).

At least one of the compound prisms 305, 310 can be rotated around theaxis 325 to dynamically adjust a steering angle between the transmitlight beam 315 and the receive light beam 320. FIG. 3B illustratesanother example of the optical beam coupling device 300 where thecompound prism 310 was rotated around the axis 325 by 180°. As shown inFIG. 3B, a receive light beam 335 is not affected by either compoundprism 305 or compound prism 310 and propagates straight through (e.g.,same as the receive light beam 320 in FIG. 3A). On the other hand, atransmit light beam 330 is deflected upward by the compound prism 305(e.g., same as the transmit light beam 315 in FIG. 3A). But, thetransmit light beam 330 is then deflected downward by the compound prism310, ending up being parallel to the axis 325 and the receive light beam335.

Note that by rotating at least one of the compound prisms 305, 310,changes in both a magnitude and a direction of desired point-ahead anglecan be achieved. FIGS. 3A-3B show that the optical beam coupling device300 can produce various magnitudes of the point-ahead angle by rotatingone compound prism relative to the other. For example, in FIG. 3A whereboth compound prisms 305, 310 are oriented identically relative to theaxis 325, deflection of the transmit light beam 315 is twice thedeflection of a single compound prism. On the other hand, in FIG. 3Bwhere the compound prisms 305, 310 are oppositely oriented relative tothe axis 325, the total deflection of the transmit light beam 330 iszero. A direction of the point-ahead angle can be also controlled byrotating both compound prisms 305, 310 together. For example, if bothcompound prisms 305, 310 were rotated 180 degrees about the axis 325relative to their positions in FIG. 3A, then the transmit light beam 330would be deflected downward when exiting the optical beam couplingdevice 300 in FIG. 3B.

In some embodiments, the wedges 307, 309 of the compound prism 305 areimplemented using different uniform materials. Similarly, the wedges311, 313 of the compound prism 305 can be implemented using differentuniforms material. In one or more embodiments, the wedges 307, 311 areimplemented using a first uniform material, e.g., SF6G05 material; andthe wedges 309, 313 are implemented using a second uniform materialdifferent from the first uniform material, e.g., fused silica material.The first uniform material (e.g., SF6G05 material) and the seconduniform material (e.g., fused silica material) may have differentdispersions. For example, the combination of SF6G05 and fused silicamaterials in each of the compound prisms 305, 310 acts as a refractiveprism that allows light of 1.53 um wavelength to propagate through thecompound prisms 305, 310 without any refraction, whereas light of 1.565um is refracted by the compound prisms 305, 310 by the total ofapproximately 51 uRad. In this embodiment, a wedge angle 317 of thewedge 307 (e.g., SF6G05 prism) is approximately 14.56 degrees and awedge angle 319 of the wedge 309 (e.g., fused silica prism) isapproximately 25.73 degrees. In this embodiment, the compound prisms305, 310 are identical, i.e., the compound prisms 305, 310 areimplemented using the same materials and their corresponding wedgeangles are same. Furthermore, three compound prisms having combinationof SF6G05 and fused silica materials can cover a field-of-view of +/−150uRad in point-ahead angle.

Referring back to FIG. 2B, in some embodiments, at least one of thefirst optical element 230 and the second optical element 235 is acompound optical element implemented using gradient-index materials ofwedged shapes. Thus, for example, the wedge 307 of the compound prism305 and/or the wedge 311 of the compound prism 310 may be implementedusing a first gradient-index material, and the wedge 309 of the compoundprism 305 and/or the wedge 313 of the compound prism 310 may beimplemented using a second gradient-index material different from thefirst gradient-index material. In some other embodiments, at least oneof the first optical element 230 and the second optical element 235 is acompound optical element implemented using gradient-index materials offlat shapes. In some other embodiments, at least one of the firstoptical element 230 and the second optical element 235 is a singleoptical element implemented using gradient-index materials of wedgedshape. In some other embodiments, at least one of the first opticalelement 230 and the second optical element 235 is a single opticalelement implemented using gradient-index materials of flat shape.

In some embodiments, instead of prisms of wedged shape (wedges), atleast one of the first optical element 230 and the second opticalelement 235 is a compound optical element that includes a pair ofdiffraction gratings for diffraction of incident light. In one or moreembodiments, the diffraction gratings of the at least one of the firstoptical element 230 and the second optical element 235 can beimplemented as transmissive gratings. In one embodiment, eachdiffraction grating of the first optical element 230 (or the secondoptical element 235) can be implemented as a ruled holographic surface.In another embodiment, each diffraction grating of the first opticalelement 230 (or the second optical element 235) can be implemented as aholographic volume, e.g., volume Bragg. In some embodiments whendiffraction gratings are used instead of wedges in the first opticalelement 230 and/or the second optical element 235, some level of spatialfiltering can be implemented to block diffracted light of higherdiffraction orders as well as undiffracted light. Additionally oralternatively, the diffraction gratings of the first optical element 230(and/or the second optical element 235) can be blazed to increaseefficiency so that most of an optical power is concentrated in aspecific diffraction order. In some other embodiments, the first opticalelement 230 is implemented as a compound prism having a pair of wedges(e.g., the compound prism 305 having the wedges 307, 309), and thesecond optical element 235 is implemented as a compound optical elementthat includes a pair of diffraction gratings. In some other embodiments,at least one of the first optical element 230 and the second opticalelement 235 is implemented as a compound optical element that includes aprism attached to a diffraction grating, e.g., a grism.

In some embodiments, differential steering between transmit and receivelight beams can be achieved based on polarization of light, instead of awavelength of light. Accordingly, the optical beam coupling device 225of FIG. 2B may further includes a pair of quarter-waveplates, whereineach quarter-waveplate is positioned adjacent to a different one of thefirst and second optical elements 230, 235. A quarter-waveplate is anoptical element that shifts polarization of received light. Aquarter-waveplate includes a polarization axis and the quarter-waveplateshifts the polarization axis 45 degrees relative to incident linearlypolarized light such that the quarter-waveplate converts linearlypolarized light into circularly polarized light. Likewise, aquarter-waveplate converts circularly polarized light incident to thequarter-waveplate into linearly polarized light. Quarter-waveplates canbe made of birefringent materials such as quartz, organic materialsheets, or liquid crystal.

FIG. 4 is an example of an optical beam coupling device 400 implementedusing a pair of quarter-waveplates 405, 410 coupled to a pair ofcompound prisms 415, 420, in accordance with one or more embodiments.The optical beam coupling device 400 may be an embodiment of the opticalbeam coupling device 225. The quarter-waveplate 405 may be configured tochange its position relative to an axis 423 together with the compoundprism 415. Similarly, the quarter-waveplate 410 may be configured tochange it position relative to the axis 423 together with the compoundprism 420. In some embodiments, the quarter-waveplate 405 is bonded tothe compound prism 415, i.e., there is no air space between thequarter-waveplate 405 and the compound prism 415. Similarly, thequarter-waveplate 410 may be bonded to the compound prism 420, i.e.,there is no air space between the quarter-waveplate 410 and the compoundprism 420.

Note that a transmit light beam 425 entering the optical beam couplingdevice 400 comprises circularly polarized light of a first handedness(e.g., right handed circularly polarized light), and a receive lightbeam 430 entering the optical beam coupling device 400 comprisescircularly polarized light of a second handedness different from thefirst handedness (e.g., left handed circularly polarized light). Thequarter-waveplate 405 may convert the transmit light beam 425 enteringthe optical beam coupling device 400 from circularly polarized light ofthe first handedness into linearly polarized light of a firstpolarization (e.g., polarized along x dimension). The quarter-waveplate410 may convert the transmit light beam 425 of the first linearpolarization back to the circularly polarized light of the firsthandedness that is output from the optical beam coupling device 400.Similarly, the quarter-waveplate 410 may convert the receive light beam430 entering the optical beam coupling device 400 from circularlypolarized light of the second handedness into linearly polarized lightof a second polarization orthogonal to the first polarization (e.g.,polarized along y dimension). The quarter-waveplate 405 may convert thereceive light beam 430 of the second linear polarization back to thecircularly polarized light of the second handedness before beingin-coupled by a receive fiber (not shown in FIG. 4).

The compound prism 415 may include a pair of wedges 407, 409 implementedusing birefringent materials for steering linearly polarized incidentlight by a specific angle depending on a direction of linearpolarization of the incident light. For example, the wedge 407 may beimplemented using a first birefringent material, and the wedge 409 maybe implemented using a second birefringent material. Alternatively, oneof the wedges 407, 409 is implemented using a birefringent material, andthe other one of the wedges 407, 409 is implemented using anon-birefringent material. Similarly, the compound prism 420 may includea pair of wedges 411, 413 implemented using same or differentbirefringent materials for steering linearly polarized incident light bya specific angle depending on a direction of linear polarization of theincident light. For example, the wedge 411 may be implemented using athird birefringent material (which may be same as the first or secondbirefringent material), and the wedge 413 may be implemented using afourth birefringent material (which may be same as the first or secondbirefringent material). Alternatively, one of the wedges 411, 413 isimplemented using a birefringent material, and the other one of thewedges 411, 413 is implemented using a non-birefringent material.

In general, the compound prisms 415 and 420 are made of birefringentmaterials that deflect (steer) the transmit light beam 425 and thereceive light beam 430 though different angles. In some embodiments,each of the compound prisms 415 and 420 can be configured tofunctionally operate as the Wollaston prism, Rochon prism, Senarmontprism, Nomarski prism, etc. The birefringent material used forimplementation of the compound prisms 415 and 420 can be e.g., calcite,quartz, sapphire, some other material, or some combination thereof. Whenthe compound prisms 415 and 420 are made of uniaxial birefringentmaterials, the c-axis of crystals can be oriented perpendicular todirection of light propagation (beam direction). The axis direction(i.e., rotation about the propagation direction) for each compound prism415, 420 (and relative to each other) is dependent on the specificdesign and polarization direction of incoming light, i.e., the transmitand receive light beams 425, 430. Thus, as result, the receive lightbeam 430 is received by the optical beam coupling device 400 along afirst LOS, and the transmit light beam 425 is output from the opticalbeam coupling device 400 along a second LOS different from the firstLOS.

In some embodiments, the optical beam coupling device 400 is implementedwithout the quarter-waveplates 405, 410. In this case, the wedges 407,409 of the compound prism 415 and the wedges 411, 413 of the compoundprism 420 are implemented using circularly-birefringent materials forsteering circularly polarized incident light by a specific angledepending of a handedness of the circularly polarized incident light.Alternatively, instead of using the compound prisms 415, 420 made ofwedges, the optical beam coupling device 400 can be implemented usingcompound optical elements based on Bragg polarization gratings. Thus,referring back to FIG. 2B, each of the first and second optical elements230, 235 may be implemented using a pair of Bragg polarization gratings.

In some embodiments (not shown in FIG. 4), instead of thequarter-waveplates 405, 410, the optical beam coupling device 400includes a pair of half-waveplates, i.e., the quarter-waveplate 405 isreplaced by a first half-waveplate and the quarter-waveplate 410 isreplaced by a second half-waveplate. A half-waveplate is an opticalelement that shifts polarization of incident light. A half-waveplateincludes a polarization axis and the half-waveplate shifts thepolarization axis 90 degrees relative to incident polarized light.Half-waveplates can be made of birefringent materials such as quartz,organic material sheets, or liquid crystal. Both transmit and receivelight beams entering the optical beam coupling device 400 including thehalf-waveplates include linearly polarized light, e.g., along directionsorthogonal to each other.

In some embodiments, a transmit light beam entering the optical beamcoupling device 400 having the pair of half-waveplates instead of thepair of quarter-waveplates 405, 410 comprises light linearly polarizedalong a first direction. A receive light beam entering the same opticalbeam coupling device 400 comprises light linearly polarized along asecond direction orthogonal to the first direction. The half-waveplatesmay change position (e.g., rotate) independently of the compound prisms415, 420 in order to line up polarizations of the transmit and receivelight beams to birefringent axes of the compound prisms 415, 420. Thetransmit and receive light beams pass through the compound prisms 415,420 without a change in their polarization. However, the transmit andreceive light beams are deflected by the compound prisms 415, 420 bydifferent angles because of a specific birefringence of each of thecompound prisms 415, 420.

In some other embodiments (not shown in FIG. 4), the optical beamcoupling device 400 further includes a pair of quarter-waveplates havingfixed positions relative to the axis 423. A first of the fixed positionquarter-waveplates may convert polarization of a transmit light beamentering the optical beam coupling device 400 from linear to circular ofa first handedness. Similarly, a second of the quarter-waveplates mayconvert polarization of a receive light beam entering the optical beamcoupling device from linear to circular of a second handedness differentfrom the first handedness. Any of the aforementioned approaches usingpolarization light can be then used in combination with the fixedposition quarter-waveplates.

FIG. 5 is a flow chart illustrating a process 500 of controlling asteering angle between a receive light beam and a transmit light beam,which may be implemented at the laser terminal 200 shown in FIG. 2A, inaccordance with one or more embodiments. The process 500 of FIG. 5 maybe performed by the components of the optical transceiver 205implemented at the laser terminal 200. Other entities may perform someor all of the steps of the process in other embodiments. Likewise,embodiments may include different and/or additional steps, or performthe steps in different orders.

The optical transceiver determines 510 (e.g., via a controller) asteering angle from a range of possible steering angles. In someembodiments, the optical transceiver determines the steering angle fromcalculation based on relative changes in position, velocity, etc.between a laser terminal comprising the optical transceiver and anotherlaser terminals. In one or more embodiments, the optical transceiverdetermines the steering angle from power reading related to the receivelight beam in-coupled by the optical transceiver.

The optical transceiver adjusts 520 (e.g., via the controller) asteering angle between the receive light beam and the transmit lightbeam to the determined steering angle. Both the receive light beam andthe transmit light beam may pass through a plurality of optical elementsof an optical beam coupling device included into the opticaltransceiver. The plurality of optical elements have a set ofcombinations for different positions of each of the optical elements,wherein each position in the set of combinations induces a differentsteering angle between the transmit light beam and the receive lightbeam. The optical transceiver controls (e.g., via the controller) aposition of each of the plurality of optical elements to adjust thesteering angle to the determined steering angle. The optical transceivercontrols (e.g., via the controller) positions of a pair of waveplates toadjust the steering angle to the determined steering angle, eachwaveplate in the pair positioned adjacent to a different one of theplurality of optical elements.

Additional Configuration Information

The foregoing description of the embodiments of the disclosure has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombinations thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Embodiments of the disclosure may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Embodiments of the disclosure may also relate to a product that isproduced by a computing process described herein. Such a product maycomprise information resulting from a computing process, where theinformation is stored on a non-transitory, tangible computer readablestorage medium and may include any embodiment of a computer programproduct or other data combination described herein.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the disclosure be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

What is claimed is:
 1. An optical beam coupling device of a firstapparatus, the optical beam coupling device comprising: a plurality ofoptical elements, wherein only a pair of identical compound prisms ofthe plurality of optical elements is configured to control a steeringangle between a first light beam and a second light beam, the firstlight beam generated by a second apparatus and received by the opticalbeam coupling device, the second light beam generated by the firstapparatus and output from the optical beam coupling device toward thesecond apparatus, each of the compound prisms comprising a pair ofwedges, and the pair of identical compound prisms having a set ofcombinations for different positions of each of the pair of identicalcompound prisms, each position in the set of combinations inducing adifferent steering angle between the first light beam and the secondlight beam.
 2. The optical beam coupling device of claim 1, wherein acompound prism of the pair of identical compound prisms is configured tosteer by a first angle the first light beam having a first wavelengthand to steer by a second angle the second light beam having a secondwavelength different from the first wavelength.
 3. The optical beamcoupling device of claim 1, wherein the wedges in each compound prismare attached to each other.
 4. The optical beam coupling device of claim1, wherein each wedge in the pair of wedges is implemented using adifferent material.
 5. The optical beam coupling device of claim 1,wherein each compound prism is implemented using a gradient-indexmaterial.
 6. The optical beam coupling device of claim 1, furthercomprising a pair of quarter-waveplates, each quarter-waveplate in thepair of quarter-waveplates positioned adjacent to a different one of thepair of identical compound prisms.
 7. The optical beam coupling deviceof claim 6, wherein: the first light beam entering the optical beamcoupling device comprises circularly polarized light of a firsthandedness; and the second light beam entering the optical beam couplingdevice comprises circularly polarized light of a second handednessdifferent from the first handedness.
 8. The optical beam coupling deviceof claim 6, wherein each quarter-waveplate is bonded to one of the pairof identical compound prisms adjacent to that quarter-waveplate.
 9. Theoptical beam coupling device of claim 1, wherein each compound prismcomprises a compound optical element implemented using a birefringentmaterial.
 10. The optical beam coupling device of claim 1, wherein: thepair of wedges in each compound prism are implemented usingcircularly-birefringent materials; the first light beam entering theoptical beam coupling device comprises circularly polarized light of afirst handedness; and the second light beam entering the optical beamcoupling device comprises circularly polarized light of a secondhandedness different from the first handedness.
 11. The optical beamcoupling device of claim 1, wherein: the optical beam coupling devicefurther includes a pair of half-waveplates, each half-waveplate in thepair of half-waveplates positioned adjacent to a different one of thepair of identical compound prisms; the first light beam entering theoptical beam coupling device comprises light linearly polarized along afirst direction; and the second light beam entering the optical beamcoupling device comprises light linearly polarized along a seconddirection orthogonal to the first direction.
 12. The optical beamcoupling device of claim 11, wherein each half-waveplate changes aposition independently from a corresponding adjacent compound prism. 13.The optical beam coupling device of claim 1, further comprising a pairof quarter-waveplates having fixed positions relative to an axis,wherein: a first of the quarter-waveplates converts polarization of thefirst light beam entering the optical beam coupling device from linearto circular of a first handedness, and a second of thequarter-waveplates converts polarization of the second light beamentering the optical beam coupling device from linear to circular of asecond handedness different from the first handedness.
 14. A method of afirst apparatus, the method comprising: determining an angle forsteering from a range of possible steering angles; and controlling aposition of at least one compound prism of a pair of identical compoundprisms of the first apparatus to adjust a steering angle between a firstlight beam and a second light beam to the determined steering angle,wherein the first light beam generated by a second apparatus andreceived by the pair of identical compound prisms, the second light beamgenerated by the first apparatus and output from the pair of identicalcompound prisms toward the second apparatus, each of the compound prismscomprising a pair of wedges, the first apparatus comprising a pluralityof optical elements, and only the pair of identical compound prisms ofthe plurality of optical elements is configured to control the steeringangle.
 15. The method of claim 14, wherein: the first light beam and thesecond light beam pass through the pair of identical compound prisms inopposite directions; and the pair of compound prisms having a set ofcombinations for different positions of each of the pair of identicalcompound prisms, each position in the set of combinations inducing adifferent steering angle between the first light beam and the secondlight beam.
 16. The method of claim 14, further comprising: controllingpositions of a pair of waveplates to adjust the steering angle to thedetermined steering angle, each waveplate in the pair of waveplatespositioned adjacent to a different one of the pair of identical compoundprisms.